This invention pertains to shields for RF coils in nuclear magnetic resonance systems.
Nuclear magnetic resonance (NMR) systems may be used as medical diagnostic tools. Generally, they consist of a region that holds the patient or sample surrounded by a set of coils. The outermost coil is usually a superconducting DC coil which provides a strong, constant magnetic field that polarizes the nuclei in the molecules of the sample or the patient's body. A smaller RF coil sits concentrically within the DC coil, and generates a time-varying RF field that excites the molecules and detects their response to this excitation. A coil assembly, generally referred to as the "gradient coil", occupies the space between the above coils and generates a time-varying audio frequency gradient field that causes the response frequency of nuclei to depend on their positions within the field. This imparted spatial information allows the generation of an image of the areas excited by the RF coil.
It is known to provide a shield between the RF coil and the gradient coil for containing the RF energy but allowing audio frequency magnetic fields to pass through. This shield may be made with two overlapping conductive layers which include insulating channels dividing the layers into conductive strips. It is also known to orient these conductive strips in the direction of the current flow induced in the shield by the RF coil. RF coil shields of this type are discussed in U.S. Pat. No. 4,642,569 of Hayes, entitled "Shield for Decoupling RF and Gradient Coils in an NMR Apparatus" (incorporated by reference).